An inertial navigation system is required to provide some indication of expected navigation performance during an alignment operation. The commonly used indicator is the current heading variance from a Kalman filter, which makes statistically optimal estimates of a quantity using observations about the quantity and the associated uncertainty in the observations. The Kalman filter also maintains an uncertainty value (standard deviation) of the error in its estimated value of the quantity. As the Kalman filter runs, the uncertainty in its estimate decreases such that the estimate converges on the “correct” value.
In a gyro-compassing alignment operation, an inertial gyro platform is aligned to true north using the rotation of the earth. Generally the alignment time is specified (e.g., four minutes is typical for military systems), and the time is extended for higher latitudes in order to meet all specified performance parameters.
During an in-motion alignment operation, the inertial gyro platform is aligned to true north using a north/east velocity aiding source from another aligned navigator, Doppler data, Global Positioning System (GPS) data, or the like. Aligning to data supplied by another navigator is often called a “transfer alignment.” For GPS-aided in-motion alignments, fixed values for the Kalman filter heading error are used to indicate “DEGRADED NAV READY” or “NAV READY” conditions. However, for in-motion alignments aboard ships, Doppler radar, or other forms of velocity-matching techniques at higher latitudes, it is often not possible to reach the level of Kalman filter heading errors, even though the system navigation capability has been reached.